Apoptosis is a term used to refer to the process(es) of programmed cell death and has been described in several cell types (Waring et al. (1991) Med. Res. Rev. 11: 219; Williams G T (1991) Cell 65: 1097; Williams G T (1992) Trends Cell Biol. 2: 263; Yonisch-Rouach et al. (1991) Nature 352: 345). Apoptosis is likely involved in controlling the amount and distribution of of certain differentiated cell types, such as lymphocytes and other cells of the hematopoietic lineage. The mechanism(s) by which apoptosis is produced in cells is incompletely understood, as are the regulatory pathways by which the induction of apoptosis occurs.